1. Field of the Invention
The present invention relates to log-periodic antennas and more particularly to an improved crossed log-periodic dipole antenna and a method of constructing same.
2. Description of the Prior Art
The crossed log-periodic dipole antenna (LPDA) consists of two orthogonally displaced indentical log-periodic dipole antennas. The crossed LPDA provides two independent, orthogonal linear polarizations. When used in a polarimeter system the purity of the two orthogonal linear polarations, which are defined by the cross-polarization level, is very important to the performance of the system. The cross-polarization level of crossed LPDAs used in these systems should be at least 20 dB below the beam peak of the matched polarization. Circular polarization can also be obtained with the crossed LPDA with the use of a 90.degree. hybrid. Circular polarization performance also depends on the cross-polarization level which should be 15 dB or more below the matched polarization. For a well constructed crossed LPDA operating in the VHF or UHF frequency range, 15 or 20 dB cross-polarization level is relatively easy to obtain. At such wavelengths the length of the smallest dipole is much greater than the diameter of the feed booms.
A crossed LPDA designed to operate at frequencies of 12 GHz or higher cannot be assembled with this conventional construction technique. At such wavelengths the diameter of the feed boom is too large compared to the length of the dipole. For high VHF and microwave frequency LPDA operation, semi-rigid coaxial cables are often used as support booms as well as the feedlines for the antenna. The dipole halves are soldered directly to the semi-rigid coaxial cables. The 0.141" and 0.085" diameter coaxial cables are often used as the feedline/boom for LPDAs in the high VHF and microwave frequencies because of their relatively small size and rigidity. For crossed LPDAs which are required to operate at 12 GHz or higher frequencies, even the small 0.085" semi-rigid coaxial cable has too large a cross section for proper performance. At such frequencies, using conventional construction techniques, the diameter of the feed booms is so large compared to the length of the smallest dipole halves that the displacement between the two smallest dipole halves is almost the same as the length of the smallest dipole half. Performance analysis of this type of antenna is based on the assumption that each dipole has two colinear dipole halves with a feed source in the center of the dipole. In other words, the displacement between the two dipole halves is assumed to be zero, and the dipole halves are free of interference from the excitation apparatus (feedline/boom). A significant displacement between dipole halves introduces undesireable cross polarization. Moreover, close proximity of the feedline/boom structure to a significant length of the dipole halves perturbs the radiation performance of the dipoles, that is, the dipole is electrically shadowed by the feedline/boom itself.
One technique for improving the performance of this antenna is to reduce the size (diameter) of the semi-rigid coaxial cable which is also used as the antenna boom. Such an approach not only reduces the displacement between dipole halves but also reduces the shadowing effect as well. A typical diameter of such cable is 0.047". However, there are two major problems in the fabrication of crossed LPDAs using 0.047" semi-rigid coaxial cables. They are:
(1) If the crossed LPDA has a high frequency requirement of 12 GHz or higher and has an operating band of 6:1, the 0.047" coaxial cable has insufficient structural rigidity to support the larger dipole elements.
(2) Since the dipole halves are soldered directly to the antenna boom, the close proximities of adjacent dipole halves make soldering nearly impossible. This is because heat transfer adversely affects the already soldered nearest dipole element. While exotic processes such as laser welding, electron beam welding or electro-plating are available to attach dipole halves to the boom simultaneously, the required assembly fixtures are elaborate, quite expensive and a new fixture is required for each new design.
A technique was developed approximately 10 years ago to solve these problems.
(1) The radiating element, which consists of dipole halves and their supporting boom, is constructed using a chemical etching process from 0.16" thick brass sheets. FIG. 1 illustrates two types of these radiating elements, one with a straight boom 1(a) and the other with a tapered boom 1(b).
(2) Two 0.047" coaxial cables are soldered to two dipole halves booms for feedline. Two copper or brass rods having a diameter similar to the diameter of the feed coaxial cable are soldered to the other two dipole half booms for symmetry.
(3) The four dipole halves, two with coaxial cables feedlines and two with brass or copper rods, are then arranged to form a crossed LPDA. The four dipole halves can be formed into two different configurations of crossed LPDAs which are shown in FIG. 2(a) and 2(b). The configuration shown in FIG. 2(a) have the coaxial cable feedlines on the inside of the dipole halves. In this configuration, the two dipole halves (which form each planar LPDA) are considerably spatially displaced. If this antenna is designed to operate up to 18 GHz, the smallest dipole half is approximately 0.16" and the displacement between the two smallest dipole halves is in the neighborhood of 0.14" which is almost the same as the smallest dipole half. The crossed LPDA structure shown in FIG. 2(b) is characterized by a straight or tapered dielectric square rod which is used to support the four dipole halves, one on each side. The size of the square rod is determined by the outline of the dipole supporting boom. The four dipole halves and supporting booms form two orthogonal pairs of balanced microstrip transmission lines. The size of the square rod and the dielectrical constant of the material used to make the rod determine the characteristic impedance of the microstrip transmission lines. In this configuration, the spacing between the smallest dipole halves is acceptable, but the proximity effect or electrical shadowing effect of the feed cable is quite large. It can be seen from FIG. 2(b) that the 0.047" coaxial cable is shadowing almost of 50% of the dipole halves. The shadowing effect pertubates the performance of this antenna. Thus, this configuration is also undesirable.
This invention is directed to an improved LPDA construction and method of making it which overcome the above mentioned disadvantages.